Cancer related fatigue (CRF) is the most common cancer side effect and can severely interfere with activities of daily living long after completionof medical treatment. Pharmacologic agents to treat CRF have been studied but there is insufficient evidence to recommend their use. Non-pharmacological interventions for CRF have also been studied but are costly to implement and involve significant patient burden. This study investigates a novel low-cost/low-burden intervention: systematic bright white light (BWL) exposure to treat CRF. Two hundred survivors of multiple myeloma and Diffuse Large B-cell Lymphoma between 100 and 160 days post-autologous stem cell transplant (ASCT) will be recruited from two medical centers and randomized to one of two conditions: BWL or dim red light (DRL), a common comparison condition in light studies. The BWL intervention consists of exposure to full spectrum bright light whereas the DRL condition will involve exposure to less intense red light (< 50 lux). Both groups will self-administer 30 minutes of light from identically appearing boxes for 4 weeks. Outcomes will be assessed through standardized subjective and objective measures at five separate time points, including baseline and follow-up. The study will specifically address recommendations made for interventions for CRF from the NCI Clinical Trials Planning meeting (JNCI, 2013).The proposed study will: 1) be the first large multisite RCT with a carefully delineated comparison condition to investigate the effects of light on CRF among ASCT survivors; 2) focus on a distinct, homogenous patient population; 3) include only survivors who experience clinical levels of CRF; and 4) address possible psychological and biological mechanisms. This RCT will have major